Heat pipe with a secondary wick for supplying subcooled liquid to high heat flux areas

ABSTRACT

A heat pipe has a sealed envelope. A primary wick extends along an inside surface of the envelope. A working fluid is contained within the envelope. The fluid is capable of undergoing a liquid/vapor phase change. An evaporator section is defined within the envelope for vaporizing the fluid. The evaporator section has at least one high heat flux area that is capable of being thermally coupled to an external heat source to be cooled by the heat pipe. A secondary wick is formed of mesh screens or sintered metal powders. The secondary wick is connected to the primary wick in the high heat flux area and connected to the primary wick at a location approximately opposite the high heat flux area, for transporting subcooled liquid to the high heat flux area. A condenser section is defined within the envelope for condensing the vaporized fluid.

FIELD OF THE INVENTION

[0001] The present invention relates to thermal control systems generally, and more specifically to heat pipes.

DESCRIPTION OF THE RELATED ART

[0002] A heat pipe comprises a sealed envelope containing an isotropic liquid-transporting wick and a working fluid capable of having both a liquid phase and a vapor phase within a desired range of operating temperatures. When one portion of the chamber is exposed to a relatively high temperature it functions as an evaporator section. The working fluid is vaporized in the evaporator section causing a slight pressure increase forcing the vapor to a relatively lower temperature section of the chamber defined as a condenser section. The vapor is condensed in the condenser section and returned through the liquid-transporting wick to the evaporator section by capillary pumping action.

[0003] U.S. Pat. No. 4,441,548 to Franklin et al. is incorporated by reference herein in its entirety for its teachings on heat pipes. Franklin et al. describe a heat pipe capable of being primed under gravity conditions, comprising a sealed envelope defining an evaporator and a condenser. An arterial tube of large liquid carrying capability is provided with an axial slot formed on the upper surface of a segment of the tube. The liquid is transported inside the arterial tube, from the condenser to the evaporator. The slot has beveled surfaces which increase the capillary pumping action to transverse wicking bodies of fine pore screen or bonded metal felt. The wicking bodies transport working fluid to or from circumferential grooves formed in the evaporator and condenser.

[0004] As the heat fluxes into the evaporator increase, the liquid inside the wick starts to boil. The resulting liquid-vapor interactions in the wick may cause dry-out to occur. This limits the heat pipe's heat flux capacity.

[0005] An improved heat pipe design that can withstand high heat fluxes in the evaporator is desired.

SUMMARY OF THE INVENTION

[0006] The present invention is a heat pipe, comprising a sealed envelope. A primary wick extends along an inside surface of the envelope. A working fluid is contained within the envelope. The fluid is capable of undergoing a liquid/vapor phase change. An evaporator section is defined within the envelope for vaporizing the fluid. The evaporator section has at least one high heat flux area that is capable of being thermally coupled to an external heat source to be cooled by the heat pipe. A secondary wick is formed of sintered metal particles. The secondary wick is connected to the primary wick in the high heat flux area and also connected to the primary wick at a location substantially opposite to the high heat flux area, for supplying subcooled liquid to the high heat flux area. A condenser section is defined within the envelope for condensing the vaporized fluid, whereby a liquid is formed.

BRIEF DESCRIPTION OF THE DRAWINGS

[0007]FIG. 1 is an isometric, partial cut-away view of a heat pipe according to a first exemplary embodiment of the invention.

[0008]FIG. 2 is a front elevation view of the heat pipe of FIG. 1.

[0009]FIG. 3 is a plan view of the heat pipe shown in FIG. 2.

[0010]FIG. 4 is a plan view of a variation of the heat pipe shown in FIG. 2.

[0011]FIG. 5 is a front elevation view of a heat pipe according to a second exemplary embodiment of the invention.

[0012]FIG. 6 is a isometric view showing the interior of the heat pipe of FIG. 5.

DETAILED DESCRIPTION

[0013] There are two very different phenomena involved in the phase change from liquid to vapor in a heat pipe evaporation: evaporation from the wick-vapor surface and nucleate boiling inside the wick. In the evaporation regime, the primary heat transfer mechanism from the wall of the envelope to the working fluid is free convection. When nucleate boiling occurs, heat transfer from the wall of the envelope by phase change becomes predominant. Of the two, boiling inside the wick is a more effective heat transfer mode. Nevertheless, the inventor has recognized that vapor formation inside the wick structure during nucleate boiling can potentially disrupt liquid transport by capillary action. In particular, bubble formation causes a problem in heat pipes with an artery for transporting fluid.

[0014] According to the present invention, a secondary wick is provided that supplies subcooled liquid to at least one high heat flux area. This allows liquid to bypass the perimeter regions in high heat flux areas where nuclear boiling occurs and intense liquid-vapor interactions exist. The secondary wick supplies ample, subcooled liquid to the highest heat flux regions. This not only increases the evaporator's heat flux capability, but also reduces the maximum evaporator temperature gradient ΔT (which is defined as the difference between the maximum evaporator wall temperature—usually at the center of a high heat input area—and the saturated liquid temperature.) Preferably, the secondary wick is formed of a porous structure that transports liquid by capillary action and does not rely on use of an artery to transport the fluid. Elimination of the artery avoids the blocking of the artery by bubbles formed under high heat flux conditions.

[0015]FIG. 1 is a partial cutaway view of a first exemplary heat pipe 100 according to the invention. The heat pipe 100 has a flat configuration. The heat pipe 100 comprises a sealed envelope or a tube 10 sealed on both ends (The ends are omitted from FIG. 1 to allow visibility into the interior of heat pipe 100.) An evaporator section 30 (the bottom in FIG. 1) is defined within the envelope 10 for vaporizing the fluid 40, whereby a vapor 45 is formed. A condenser section 35 (the top in FIG. 1) is defined within the envelope 10 for condensing the vaporized fluid 45, whereby a liquid 40 is formed. The primary wick is an internal isotropic capillary pumping structure such as a wick 25, extending between the evaporator section 30 and the condenser section 35. The primary wick 25 typically comprises a fine mesh screen fitted tightly to the wall of the envelope 10. Sintered metal powder wicks can also be used as the primary wick 25. Preferably, the primary wick is sufficiently thin in the high heat flux area 11 to reduce the evaporator thermal resistance.

[0016] The evaporator section 30 has at least one high heat flux area 11 that is capable of being thermally coupled to an external heat source 12 (which may be, for example, an electrical device) to be cooled by the heat pipe 100. The evaporator 30 further includes a secondary wick 26 formed of mesh screen or sintered metal powders. The secondary wick 26 is connected to the primary wick 25 in the high heat flux area 11, and is also connected to the primary wick 25 at a location “substantially opposite” to the high heat flux area, for transporting subcooled liquid directly from the condenser section 35 to the high heat flux area 11. In FIGS. 1 and 2, the secondary wick 26 is preferably connected to the primary wick 25 at a location 13 about 180 degrees from the high heat flux area 11.

[0017] During experiments, a flat configuration heat pipe with a point heat source was observed. Without the secondary wick 26, a dry-out pattern was observed in the center of the high heat flux area 11. Over time, the dry-out region expanded out towards the edges. In the dried-out area the primary heat transfer mechanism is no longer the phase change, but is merely natural convection, which has a lower heat transfer capability. When the secondary wicks 26 were added, there was no dry out pattern.

[0018] Although the exemplary embodiment includes three columns 26 as the secondary wick, the secondary wick may have a variety of shapes, and may vary in number. For example, one or more columns 26 may be included. The number and size of the secondary wick columns can be optimized (either empirically or using computational fluid dynamics and heat transfer simulation, for example, a program for finding a solution satisfying the Navier-Stokes equation), to maximize the liquid supply to all regions and minimize the loss of surfaces for phase change heat transfer. For example, given the heat input Q and surface area, a system liquid pressure drop can be calculated. Assuming that 90% of the liquid to the high heat flux area 11 is to be supplied via the secondary wick 26, a liquid delivery rate is determined. The size and number of secondary wick columns can be selected, taking into account the desired liquid delivery rate, the porosity and permeability of the wick 26.

[0019] Referring now to FIGS. 3 and 4, two different shapes for the columns 26 are shown. In FIG. 3, the columns 26 are circular porous cylinders. In FIG. 4, the columns 26′ are flat webs or parallelipipeds, with rectangular cross sections. Other shapes, such as described below with reference to FIGS. 5 and 6 may also be used. When using column structures for the secondary wick(s), it is preferred that a column be positioned substantially at the center of the high heat flux area 11. One of ordinary skill understands that the center may be the centroid of the high heat flux area 11. However, if the high heat flux area 11 has a non-uniform heat input distribution, then the column 26 may be located at the location of the centroid of the heat input in the high heat flux area.

[0020] The secondary wick is formed from a sintered metal material, such as powders of copper, aluminum, nickel, sodium, potassium, silver or lead, or a sintered non-metal material such as silicon, silicon carbide or silicon nitride. Advantageously, the sintered structure of secondary wick(s) 26 eliminates the problem of bubble formation by having small pore radii. The fluid travels along the porous structure of the column. Fine mesh screens can also be used to form the secondary wick columns 26.

[0021] In use, an external heat source 12 is thermally coupled to a high heat flux area 11 of a portion of the heat pipe 100 containing the evaporator section 30. Subcooled liquid is transported to the high heat flux area 11 from a directly opposite portion 13 of the evaporator section 30 via a secondary wick 26 formed of fine mesh screens or sintered metal pwerders. The transfer of heat energy Q occurs when the evaporator section 30, exposed to a relatively high temperature or a heat source 12, produces a vaporization of a working fluid 40 capable of having a liquid/vapor phase change. A major portion of the liquid/vapor phase change occurs by nucleate boiling in the high heat flux area 11, instead of solely evaporation at the liquid surface. A slight pressure increase results from the vaporization of the fluid 40 within the evaporator section 30 whereby the vapor 45 flows through the interior vapor space 50 of the heat pipe 100 to the relatively cooler, lower pressure condenser section 35 which rejects heat to some external heat sink (not shown). The vapor 45 is condensed in the condenser section 35 and returned through the wick 25 to the evaporator section 30 by capillary action.

[0022]FIGS. 5 and 6 show a second exemplary heat pipe 200 according to the invention, wherein the secondary wick 226 is a sintered body having a cylindrical core 228 and at least two radial web portions or wick blocks 227 connecting the core to the primary wick 225. In FIG. 6, the end of the heat pipe 200 is omitted from the drawing to more clearly show interior components, but it is understood that an end is included in the actual device. In heat pipe 200, the secondary wick 226 extends longitudinally from the evaporator section 230 to the condenser section 235. In this configuration, the wick core 228 transports the majority of the liquid from the condenser to the evaporator.

[0023] The diameter of the wick core 228 can be determined using the conventional capillary limit calculation. The size of the core diameter is estimated. A liquid pressure drop calculation is performed. If the pressure drop is below the capillary pressure in the evaporator wick, the size of core 228 can be reduced, and the pressure drop increases. A smaller sized core 228 is preferred because a smaller core leaves more vapor space, improving the efficiency of the heat pipe. As long as the total pressure drop is below the capillary pressure, the core size is acceptable. The wick blocks 227 may have small openings in the vicinity of the evaporator 230 and/or the condenser 235, so that the vapor can become more evenly distributed around the circumference of the heat pipe 200.

[0024] The width and number of the wick blocks 227 may be optimized using the same principles applied to the embodiment of FIG. 1. Although the exemplary embodiment shows an even number of wick blocks 227, including a wick block diametrically opposed to the high heat flux area 211, an odd number of blocks may be used, in which case one or two wick blocks would be approximately opposite the high heat flux area, but not 180 degrees away.

[0025] In heat pipe 200, the cylindrical core 228 is porous. Instead of using wick columns 26, the plurality of wick blocks or webs 227 feed the evaporator primary wick 225 with fluid conducted from the wick core 228.

[0026] Advantageously, the secondary wick 226, including the core 228 and wick blocks 227 can be sintered with a single mandrel (not shown), for example using techniques described in U.S. Pat. No. 4,274,479 to Eastman, which is incorporated by reference herein in its entirety.

[0027] By using a secondary wick 226 to supply liquid, the main wick 225 in the evaporator 230 can be optimized for boiling/evaporation heat transfer without having to also consider the capillary liquid transport in primary wick 225. Therefore, the main wick 225 can be made thinner to allow easier escape of vapor bubbles generated near the inner wall of envelope 210. Exemplary primary wick thicknesses may vary between 0.01 inches (0.025 centimeter) and 0.09 inches (0.23 centimeter). Similarly, a thinner wick in the condenser 235 provides a smaller condensation thermal resistance.

[0028] Although the invention has been described in terms of exemplary embodiments, it is not limited thereto. Rather, the appended claim should be construed broadly, to include other variants and embodiments of the invention which may be made by those skilled in the art without departing from the scope and range of equivalents of the invention. 

What is claimed is:
 1. A heat pipe, comprising: a sealed envelope; a primary wick extending along an inside surface of the envelope a working fluid contained within the envelope; the fluid being capable of undergoing a liquid/vapor phase change; an evaporator section defined within the envelope for vaporizing the fluid, the evaporator section having at least one high heat flux area that is capable of being thermally coupled to an external heat source to be cooled by the heat pipe; a secondary wick formed of mesh screens or sintered metal or non-metal particles, the secondary wick being connected to the primary wick in the high heat flux area and also connected to the primary wick at a location approximately opposite to the high heat flux area, for transporting subcooled liquid to the high heat flux area; and a condenser section defined within the envelope for condensing the vaporized fluid, whereby a liquid is formed.
 2. The heat pipe of claim 1, wherein the secondary wick has a cylindrical core and at least two radial web portions connecting the core to the primary wick, and the primary wick is attached to the inner surface of the envelope.
 3. The heat pipe of claim 2, wherein the secondary wick extends longitudinally from the evaporator section to the condenser section.
 4. The heat pipe of claim 2, wherein the cylindrical core is porous.
 5. The heat pipe of claim 1, wherein the secondary wick is located substantially at a center of the high heat flux area.
 6. The heat pipe of claim 5, wherein the secondary wick has a cylindrical shape.
 7. The heat pipe of claim 5, wherein the secondary wick has a flat web shape.
 8. The heat pipe of claim 1, wherein the primary wick is sufficiently thin to promote heat transfer in the high heat flux area.
 9. The heat pipe of claim 1, wherein the secondary wick is formed of one of the group consisting of powders of copper, aluminum, steel nickel, silicon, silicon carbide and silicon nitride.
 10. A heat pipe, comprising: a sealed envelope; a primary wick extending along an inside surface of the envelope a working fluid contained within the envelope; the fluid being capable of undergoing a liquid/vapor phase change; an evaporator section defined within the envelope for vaporizing the fluid, the evaporator section having at least one high heat flux area that is capable of being thermally coupled to an external heat source to be cooled by the heat pipe; a secondary wick formed of mesh screens or sintered metal or non-metal particles, the secondary wick having a cylindrical core and at least two radial web portions connecting the core to the primary wick, the secondary wick being connected to the primary wick in the high heat flux area, for transporting subcooled liquid to the high heat flux area; and a condenser section defined within the envelope for condensing the vaporized fluid, whereby a liquid is formed.
 11. The heat pipe of claim 10, wherein the secondary wick extends longitudinally from the evaporator section to the condenser section.
 12. The heat pipe of claim 10, wherein the cylindrical core is porous.
 13. The heat pipe of claim 10, wherein the primary wick is sufficiently thin to promote heat transfer in the high heat flux area.
 14. The heat pipe of claim 10, wherein the secondary wick is formed of one of the group consisting of powders of copper, aluminum, steel nickel, silicon, silicon carbide and silicone nitride.
 15. In a heat pipe having a sealed envelope, a primary wick extending along an inside surface of the envelope, a working fluid contained within the envelope, the fluid being capable of undergoing a liquid/vapor phase change, an evaporator section defined within the envelope for vaporizing the fluid, and a condenser section defined within the envelope for condensing the vaporized fluid, a method for transferring heat, comprising the steps of: (a) thermally coupling an external heat source to a high heat flux area of a portion of the heat pipe containing the evaporator section; and (b) transporting subcooled liquid to the high heat flux area via a secondary wick formed of mesh screens or sintered metal, the secondary wick being connected to the primary wick in the high heat flux area and also connected to the primary wick at a location approximately opposite to the high heat flux area.
 16. The method of claim 15, wherein step (b) includes transporting subcooled liquid in a direction perpendicular to a longitudinal axis of the heat pipe, to the high heat flux area.
 17. The method of claim 15, wherein step (b) includes conducting subcooled liquid to the high heat flux area from a porpous central cylindrical core portion of the secondary wick extending from the condenser section to the evaporator section, via a web portion of the secondary wick connecting the core to primary wick.
 18. The method of claim 15, further comprising boiling liquid inside the primary wick within the high heat flux area. 